Telephone repeater



Patented Nov. 23, 1926'. 1

UNITED STATES PATENT OFFICE.

CHARLES W. GREEN, OF MILBURN, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TELEPHONE REPEATER.

Application filed June 20,

This invention'relates to telephone repeaters, and has for its object to adjust the gain of a repeater to compensate for attenuation in the interconnected lines, and in particular to effect this compensation where the attenuation is not uniform with frequency.

In the form of the invention described herein, a two-stage vacuum tube repeater is provided with two gain regulating trans being substantially suppressed above this frequency, and the attenuation increases 25 rather sharply throughout a small range just below the cutoff frequency.

In accordance with this invention the gain of the repeater is made to increase in a manner complementary to the increase in attenuation substantially up to the cutoff frequency of the line.

Further objects of the invention Wlll appear from the followingdetailed description and claim taken in connection with the accompanying drawings, in which Fig. represents diagrammatically a repeater circuit embodying the invention, Fig. 2 shows graphically the relation between line attenuation and repeater. gain and F 1g. 3 1s a similar view showing repeater gain characteristics with different adjustments.

Referring to the drawings by reference characters, repeaters A and B are shown connected in the oppositely directed channels of a four-wire transmission system,..repeater A transmitting currents from west to east and repeater B transmitting inthe opposite direction. Repeater A will be described in detailwith the understanding that repeater B is substantially the same except for slight difierences in the battery connections, which will be pointed out hereinafter.

The line section on the input side of One of these transformers, which is In medium heavy load-- ed lines, the line has a cutoff frequency n the" neighborhood of 2400 cycles, transmission 1923. Serial No. 646,523.

repeater A is connected through transformer 12 to the vacuum tube 14, which is of the type having a high voltage output. Tube 14; is connected by a transformer 16 to a vacuum tube 18 of the type having a large current output. A transformer 20 connects the output electrodes of vacuum tube 18 to a line section21.

Bridged across the terminals of line 10 is a resistance 23 of a value substantially equal to the characteristic impedance of the line or to such impedance as viewed through a non-unity ratio transformer, not shown. Since the impedance of the repeater itself is high compared to resistance 23, the effect of the resistance is to cause the repeater to present to the line a substantially constant resistance.

The secondary of input transformer 12 is provided'with taps 25, any one of which may be connected to the grid of vacuum tube 14. These taps are so spaced as to cause a change in the gain of the repeater of five miles of standard cable. The secondary of transformer 16 is provided with similar taps 27 to be connectedby the dial switch 28 to the grid of vacuum tube 18. Transformer 16 has gain adjustin steps of one mile of standard cable, and t e total variation in gain is much less than that obtainable by transformer 12. This arrangement prevents overloading the vacuum tube 14 by reducing the transmission too much at the inter-stage position.

In series with the primary of transformer 12 is an inductance 30of a value to resonate with the total capacity across the secondary of the transformer at a frequency near the upper limit of the range of frequencies transmitted by the line. This capacity includes the grid, filament capacity of tube 14, the capacity between the secondary turns, and that of a condenser 31, which is connected across the total number of turns in the secondary. A condenser 33 is adjustably connected inshunt to a portion of inductance 30. This parallel circuit is adjusted to be anti-resonant at a frequency slightlyabove the frequency of resonance above mentioned. The effect of this antiresonant circuit is to cause the reactance of inductance 30 to increase rapidly as the frequency approaches the frequency at which comprises two networks in series with each other, the first of which comprises inductance 30 and condenser 33, and is inductlvely reactive'throughout the frequency range to be transmitted, and the second of which comprises the mutual of transformer 12 in parallel with the capacities mentioned above and has a capacitative reactance throughout the upper part of the frequency band. These networks are given values such that they are series resonant at a frequency at which maximum transmission is desired. The voltage operated vacuum tube 14 is connected across the second network or any desired part thereof so'that it receives its maximum voltage at the resonant frequency. Any leakage inductance which may be present in transformer 12 is effectively in series with the inductance 30 and its effect is merely additive.

In Fig. 2 the full line-curve represents loss in standard miles with frequency in 100 geographical miles of medium heavy loaded cable. The irregularity in attenuation shown by this curve would cause distortion. of the telephone currents if not corrected for. In the dotted line curve is shown the gain characteristic of a repeater which will equalize the transmission up to the cutoff frequency of the line. Above this freuency the gain of the repeater falls off s harply. This is due partly to the antiresonant effect of the loop comprising condenser 33, and partly because these frequencies are; above the series resonance frequency above described. While the curve showmggain of the repeater is shown adjacent the loss curve, considerable variation in absolute position is, ofrcourse, permissible s1nce the transmission at any point in a long line may vary within fairly liberal limits from the level at the beginning of the line.

Inductance 30 and the connection thereto of condenser 33 are adjustable so that ,the repeater gain characteristic can be varied to equalize differing attenuation characteristics of lines to be. connected thereto.- In Fig. 3.is shown a group of curves showing gain characteristics obtained by adjusting these elements. The curves having peaks at higher frequencies are obtained by decreasing inductance 30 or the portion thereof shunted by condenser 33 or by decreasing both. These last mentioned gain characteristics are suitable for .equalizing the attenuation in a phantom circuit comprising medium heavy loaded cable. Suitable values for the tuning elements, With'a typical input transformers to the plates of the respective tubes. Condensers 39 and 40 provide paths,

for the alternating component of the output current. These condensers and inductances form filters which permit the passage of direct current but prevent the fiow of alter: nating current. This'serves not only to prevent battery fluctuations from reaching the tubes, but also prevents cross-talk therebetween. Since the output current from tubes 18 and 18 may be large, this current is caused to flow in parallel through two cons densers 40 connected to opposite terminals of the filaments so as to avoid fluctuations in the.

filament current, which might produce crosstalk in other circuits. The four filaments are connected in series with each other and with a source of current'42, and an'inductance 44 may be also inserted to prevent fluctuations in the filament current. Suitable negative potentials for the grids offtubes 14 and 14' are obtained by connecting the grids to points on resistances 46 in shunt to the filaments of tubes 18 and 18', which are at lower potentials than'the filaments of tubes 14 and 14, respectively. N egative potentials for the grids of tubes 18 and 18 are obtained by batteries 48.

While the invention has been shown in a single embodiment thereof, it is obvious that the various features of. the invention are capable of wide application within the scope of the appended claims.

What is claimed is:

1. .A circuit for amplifying waves of' a given frequency, said circuit comprising, in

combination, a vacuum tube amplifier,; a

transformer connected to the input electro-desthereof, an inductance in series with the primary of said transformer, and a condenser of fixed value in shunt to the secondary of said transformer and of such value as to form with said transformer and said tube a circuit resonant with said inductance at said frequency.

2. In combination in a circuit for transmitting waves of a band of frequencies comprising a vacuum tube amplifier having input electrodes, a transformer having a primary winding and a secondary winding connected to said input electrodes, an inductance in series with the primary of said transformer, a condenser in shunt'to a portion of said inductance, and a condenser in trodes, a transformer havin shunt to the secondary winding of said transformer, said inductance having a value to resonate with the total capacity across said secondary winding at a frequency near the upper limit of said band of frequencies.

3. In combination in a circuit for transmitting waves of a band of frequencies, a vacuum tube amplifier having input eleca primary winding and a secondary winding connected to said input electrodes, an inductance. in series with said primary winding of said transformer, a condenser in shunt to a ortionof said inductance, said inductance aving a value to resonate with the total capac ity across said secondary winding at a. frequency near the upper limit of saidband of'frequencies, the circuit com rising said portion of said inductance an said shunt condenser being adjusted to be anti-resonant at a frequency above said resonant frequency near the upper limit of said band.

4. The combination as set forth in claim 3 in which said inductance is adjustable and said condenser is adjustably connected thereto. p

5. A circuit for transmitting waves of a band of frequencies, comprising in combinaworks, the other of said networks being tion, a vacuum tube amplifier having input electrodes, means for increasing the gain of said amplifier at the higher frequencies of said band of frequencies to be transmitted, comprising two networks in series resonance at a frequencynear' the upper end of sald' band, said input elect odes beingconnected across at least a porti n of one of said netitransformer therefor, a condenser of former having an inductance effectively in series with its primary winding for resonating with the capacity effectively across its secondary winding at a frequency in the neighborhood of the upper frequencies of said range, and taps on said secondary, the

input electrodes of said vacuum tube being a ustably connected to said secondary by means of said taps.

7.- A vacuum tube amplifier circuit comprising two vacuum tubes arranged in tandem, an input transformer for the first tube and an inter-stage transformer between said tubes, the secondaries of each of said transformers being provided with taps, and means for adjustab of the respective tubes to said taps for controlling the gain of the repeater, the total amplifier circuit gain change obtainable by means of said interstage transformer taps being small in comparison to thatobtainable by means of said input transformer taps, whereby overloading of said first tube, due to excessive gain reduction by means of said interstage transformer taps, is prevented.

In witness whereof, I hereunto subscribe myname this 18th day of June A. D., 1923.

. CHARLES W. GREEN.

y connecting the grids 

